1. Technical Field
The present invention relates to a semiconductor laser diode structure and, more particularly, to a laser diode structure which includes compensation for voltage-dependent parasitics associated with the laser diode junction.
2. Description of the Prior Art
The concept of transmitting several television channels over a single-mode optical fiber via analog intensity modulation of a semiconductor laser diode has been receiving considerable attention. As proposed in the prior art, an arrangement would transmit multi-channel amplitude modulated vestigial side-band (AM-VSB) signals, as used in present day common antenna (i.e., cable) television (CATV) systems, in an optical fiber transmission media. Such an arrangement would be useful in a CATV trunk system, a fiber-to-the-home network, or the like. Optical fiber transmission systems that use frequency division multiplexing overcome compatibility problems and have advantages such as simplicity of design, reduced bandwidth requirements for lightwave components, and much lower cost, as compared with optical time division multiplex (TDM) systems.
The low loss of optical fibers make analog sub-carrier modulation an attractive technology. Several signals at different sub-carrier frequencies, each signal representing one of the analog/data channels to be multiplexed, are summed and applied concurrently to the input of the laser device. The resulting laser drive current is a dc bias level plus the set of sub-carrier signals. For the laser, the magnitude of the optical output power from the laser is an approximately linear function of its drive current. The resulting sub-carrier frequency division multiplexed (FDM) output signal is applied to an optical fiber for transmission over an extended distance.
Multi-channel signal transmission requires special limitations on the power, linearity and intensity noise of the transmitting laser diode. For adequate system performance, the laser output light intensity must be a linear function of its drive current. Strict limitations on laser diode linearity are required, for example, because of the wide dynamic range of the National Television Systems Committee (NTSC) standard video format. Lasers with fairly linear characteristics are available with composite second and third harmonic distortion down to -30 dBc and -40 dBc, respectively, from the relevant carrier fundamental for acceptable levels of input modulation current. In that NTSC standard video format, the ratio of the magnitude of the total composite of the third order intermodulation distortion products at the carrier frequency to the magnitude of the carrier must be less than approximately -60 dBc. Similarly, the composite second-order (CSO) distortion, i.e., the sum of several tens of two-tone products (or the ratio of the largest composite second-order peak to the carrier), must be less than approximately -50 dBc. This low distortion must be obtained when the laser is modulated with an optical modulation depth of typically one to ten percent per channel, to insure acceptable carrier-to-noise performance.
To obtain such high signal quality in view of the large number of distortion products, the transmitting laser light power versus drive current characteristic curve must be extremely linear. There are several known causes of nonlinearity in semiconductor laser diodes. Some of the causes are high frequency relaxation oscillations, low frequency heating effects, damping mechanisms, such as gain compression and nonlinear absorption, and leakage current. The resulting effect of the distortion is interference in each of the signals, due to the presence of the remaining signals.
Various methods have been used in the past to control the linearity of laser diodes for analog modulation applications. U.S. Pat. No. 4,101,847 issued to A. Albanese on Jul. 18, 1978 discloses a control circuit which automatically adjusts the operating current level of a laser diode to maintain linearity of the output. In particular, a test signal is used to modulate the laser and the operating current level is adjusted until the harmonic distortion component at twice the test signal frequency (i.e., second-order harmonic) is minimized. In an alternative technique, U.S. Pat. No. 5,012,484 issued to E. J. Flynn et al. on Apr. 30, 1991 teaches a laser diode device fabricated to produce a non-uniform photon density in the laser cavity, with the density of photons being larger in the rear portion of the cavity than in the front portion. During the operation of the laser, the gain in the back portion is therefore substantially independent of the laser current, whereas the gain in the front portion is a function of the laser current. The non-uniformity results in a greater likelihood that the best operating current level will be substantially greater than the threshold current, with a concomitant higher output power.
As mentioned above, one source of laser nonlinearity is the leakage current present around the active region of the laser diode structure. U.S. Pat. No. 5,020,049 issued to G. E. Bodeep et al. on May 28, 1991 discloses a technique for providing a laser diode with a relatively low leakage current (and hence, relatively linear response) by operating the laser diode at a bias current level associated with the point of inflection of the L-I curve (output power vs. operating current). Operation with the laser biased at or near this point results in extremely linear performance, even though the laser may have insufficient blocking of leakage current to otherwise produce acceptable linearity.
A remaining issue in the area of laser diode nonlinearity is that there appears to be a relatively small acceptable operating current range which provides the required linear relationship. Prior art techniques thus address various means of controlling the operation of the laser to remain in the vicinity of the optimum bias current and often limit the number of fabricated lasers which meet the requisite criteria. A need remains, therefore, for a means of improving the inherent linearity of the laser diode such that controls as those of the prior art are not required.